The present invention relates to a Mach-Zehnder electro-optic modulator and, more particularly, to a Mach-Zehnder electro-optic modulator comprising a lithium niobate substrate of alternating domains of appropriate length and number to compensate for device chirp caused by phase differences in the split optical signals passing therethrough.
For high bit rate, long haul communications via optical fibers, appropriate light sources serving as the transmitter are essential. Semiconductor lasers can be directly modulated at high speeds and, consequently, are used extensively in optical transmission systems. However, as the demand for increasingly higher bit rates grows (i.e.,  less than 10 Gb/s, and in some cases, 40 Gb/s). certain inherent properties of semiconductor lasers come into play. One of these inherent properties is xe2x80x9cchirpingxe2x80x9d-defined as a change in the transmission wavelength as the laser is modulated with a high bit rate input signal.
Better control of transmitter chirp can be realized by using so-called xe2x80x9cexternalxe2x80x9d modulation in place of xe2x80x9cdirectlyxe2x80x9d modulating the semiconductor laser device. In external modulators, a conventional 1.5-1.6 xcexcm semiconductor laser is operated in continuous wave (CW) mode and the continuous output from the laser is externally switched xe2x80x9conxe2x80x9d and xe2x80x9coffxe2x80x9d using an optical modulator to provide the desired high bit rate (binary) optical signal. Often, a Mach-Zehnder interferometer is used as the optical modulator. A Mach-Zehnder interferometer comprises a pair of waveguide channels (also referred to as xe2x80x9carmsxe2x80x9d) connected between an optical waveguide splitter and an optical waveguide combiner. An optical source (such as the CW laser diode) is coupled into the waveguide splitter, which serves as a Y-branch splitter or directional coupler. The two light beams from the splitter propagate through the pair of waveguide channels and are reunited (combined) by the waveguide combiner. Electrodes are disposed over each arm of the pair of waveguide channels and, by providing modulating voltages to one or both electrodes (i.e., the input xe2x80x9cdataxe2x80x9d on/off signal), the relative phases of the two light beams may be altered. In most conventional arrangements, one electrode is held at ground potential and the other is modulated with an electrical RF data signal.
While the use of an external modulator has been found to allow for higher bit rates to be achieved, the lithium niobate substrate material used to form most high speed conventional external modulators can exhibit a phase difference between the two arms of the modulator, introduced by different electric fields being associated with each of the arms. That is, the applied voltage to the electrodes will change the electric field (by different amounts) in the lithium niobate substrate material directly underneath both the RF electrode and the ground plane. With a different electric field on each waveguide channel, therefore, the effective index changes in each arm will differ, introducing a phase difference (i.e., xe2x80x9cchirpxe2x80x9d) into the output signal.
Thus, a need remains in the art for a Mach-Zehnder electro-optic modulator arrangement that is capable of providing the high bit rate throughput necessary for current and future communication applications, yet does not introduce an undesired amount of chirp into the optical output signal.
The need remaining in the prior art is addressed by the present invention, which relates to a Mach-Zehnder electro-optic modulator and, more particularly, to a Mach-Zehnder electro-optic modulator comprising a lithium niobate substrate of alternating ferroelectric domains to compensate for optical phase differences between the optically split signals passing through the two arms of the Mach-Zehnder modulator.
In accordance with the present invention, a single-ended Mach-Zehnder electro-optic modulator is formed to include three separate regions disposed in tandem. The first region is similar to a conventional Mach-Zehnder electro-optic modulator and includes a pair of spaced-apart arms, with a xe2x80x9cground planexe2x80x9d electrode disposed over a first waveguide and an RF electrode carrying the modulating signal (also referred to as the xe2x80x9chotxe2x80x9d electrode) disposed over the second, remaining waveguide. A second, relatively short, transition region is disposed immediately after the first region. The electrode configuration is transitioned in this second region so that the RF electrode is positioned over the first waveguide at the output of the second region and the ground plane is positioned over the second waveguide. A third region, comprising lithium niobate substrate material having its domain inverted with respect to the first region, is disposed after the second region, where the RF electrode is maintained over the first waveguide and the ground electrode over the second waveguide.
The arrangement of the present invention results in introducing a positive xe2x80x9ctotal optical path length differencexe2x80x9d (TOPD), also referred to as chirp, between the pair of optical signals exiting the first region (which would result in signal chirp if the signals were recombined and the device output taken at this point). However, an opposite TOPD is exhibited in the third region. Therefore, with proper control of the combination of the first, second and third regions, the TOPD can be made to be essentially zero. As long as the optical path lengths of the waveguides in the first and third regions are essentially the same, the positive and negative values of TOPD will cancel and essentially eliminate most of the chirp in the optical output signal. The use of the domain inverted lithium niobate substrate in the third region preserves the modulation on the optical output signal, since the electrode location is switched between the first and third regions. Therefore, the combination of moving the RF electrode with inverting the domain in one region of the modulator results in a Mach-Zehnder device with greatly reduced chirp.
In another embodiment of the present invention, a pre-defined amount of xe2x80x9cchirpxe2x80x9d (which is useful in some applications) can be introduced by modifying the physical length of either the first region, the third region, or both the first and third regions.
It is an aspect of the present invention that multiple transition and xe2x80x9cdomain invertedxe2x80x9d regions can be cascaded along the length of the optical substrate to form a device which alternates between xe2x80x9cpositivexe2x80x9d TOPD and xe2x80x9cnegativexe2x80x9d TOPD, allowing for improved control of the amount of chirp in the optical output signal.
Other and further aspects and embodiments of the present invention will become apparent during the course of the following discussion and by reference to the accompanying drawings.